1. Field of the Invention
The present invention is directed to a method and apparatus for extracting position information and activity information from a composite signal generated by a motion sensor. The present invention is particularly applicable to implantable medical devices, such as pacemakers and defibrillators, but the invention is generally applicable any system that utilizes a motion sensor to generate a composite signal containing information about position and activity.
2. Background
Pacemakers are small medical devices that can be implanted in a person with heart disease to provide periodic electrical pulses to stimulate regular beating of the patient""s heart. Over the years many improvements have been made to the first pacemaker designs. One such improvement was the addition of a rate adaptive feature. A rate adaptive pacemaker attempts to adjust the rate at which pacing pulses are delivered to the heart to meet the patient""s physiological needs.
One method of providing adaptive rate pacing that has proven fairly successful involves use of a motion sensor to determine the patient""s level of physical activity. A commonly used motion sensor is an accelerometer, which generates an electrical signal proportional to acceleration. Many accelerometer designs have been developed over the years and have been used in pacemakers to sense the physical activity level of a patient by sensing, for example, the footfalls of a patient as he or she walks, climbs stairs, jogs, or runs. Various methods have been devised for processing the electric signal generated by an accelerometer to determine the physical activity level of the patient.
Clinical studies have shown, however, that a better prediction of the pacing rate needed by a patient can be obtained by considering not only the physical activity level of the patient but also the position of the patient. For example, if a person is walking, the rate at which the person""s heart should be paced depends not only on the sensed level of physical activity but also on whether the patient is walking up hill, down hill, or on a level surface. As another example, it is believed that a healthy heart naturally beats slower while the person is lying down than while the person is sitting or standing.
Several schemes have been developed to determine both the patient""s activity level and the patient""s position. For example, U.S. Pat. No. 5,649,968 to Alt et al. (the ""968 patent) discloses a method in which the morphology of a signal generated by a motion sensor is analyzed to determine the patient""s position. According to the inventors of the ""968 patent, clinical studies show identifiable differences in the morphology of activity signals that depend on the position of the patient. U.S. Pat. No. 5,865,760 to Lidman et al. looks for changes in electrocardiogram data that indicate a change in the patient""s position. U.S. Pat. No. 5,593,431 to Sheldon (the ""431 patent) discloses use of a so called DC accelerometer that creates a composite signal containing information about both the position and the activity level of the patient. The ""431 patent, however, does not disclose an efficient way to separate the position information from the activity level information in the composite signal.
The instant invention is directed to a method and apparatus for extracting position information and activity information from a composite signal generated by a motion sensor. An analog to digital converter converts the composite signal from analog to digital format. The digitized signal is passed through a low pass filter with a frequency response of about zero hertz to about ten hertz, and the digitized signal is also passed through a narrow low pass filter with a frequency response of about zero hertz to about 0.5 hertz. The output of the narrow low pass filter is subtracted from the output of the low pass filter. The resulting difference represents information regarding the activity level of the motion sensor, and the output of the narrow low pass filter represents information regarding the position of the sensor.
In an alternative embodiment, a sigma-delta converter (also known as a deltasigma converter) changes the sensor signal from analog to digital format. A sigma-delta converter typically includes a modulator, one or more downsample stages, and one or more anti-alias filters. The modulator typically converts an analog signal into a digital format, and the downsample stages and anti-alias filters typically convert the digital signal into a format that is useful for the particular application. The digitized signal is passed through a narrow low pass filter with a frequency response of about zero hertz to about 0.5 hertz. The output of the narrow low pass filter is subtracted from the output of sigma-delta converter. The resulting difference represents information regarding the activity level of the motion sensor, and the output of the narrow low pass filter represents information regarding the position of the sensor.